1. Field of the Invention
The present invention relates to position indicating systems generally and particularly to position indicating circuits for indicating the position of a nuclear reactor control rod.
2. Description of the Prior Art
Position indicating systems are generally known for indicating the position of a control rod in a nuclear reactor. Such known systems employ a permanent magnet mounted to the control rod to be movable internally of the reactor along with the control rod. A reed switch circuit is usually located externally of the control rod housing along the operating length of the control rod and hence along the operating length of the permanent magnet. As the control rod moves, different reed switches of the circuit are actuated by the magnetic flux of the permanent magnet and yield appropriate output signals whose magnitudes are indicative of the position of the control rod. These output signals are used to control the nuclear reactor.
The reed switches of such reed switch circuits have certain drawbacks such as an inherent hysteresis characteristic and the tendency to stick in the closed condition. The hysteresis problem results in an uncertainty as to the actual position of the control rod while a stuck reed switch provides an erroneous indication of the position of the control rod. To eliminate or minimize the hysteresis problem a higher density of reed switches is sometimes used with the outputs of at least two reed switches being energized per control rod position and these outputs then being averaged to provide a more accurate position indication. To eliminate the stuck reed switch problem, back-up reed switches are provided or the reed switch contacts are intermittently exercised by applying a strong alternating flux field to the reed switch to oscillate the contacts.